From: "Michael S. Tsirkin" <mst@redhat.com>
To: Thiago Jung Bauermann <bauerman@linux.ibm.com>
Cc: Mike Anderson <andmike@linux.ibm.com>,
Michael Roth <mdroth@linux.vnet.ibm.com>,
Jean-Philippe Brucker <jean-philippe.brucker@arm.com>,
Jason Wang <jasowang@redhat.com>,
Alexey Kardashevskiy <aik@linux.ibm.com>,
Ram Pai <linuxram@us.ibm.com>,
linux-kernel@vger.kernel.org,
virtualization@lists.linux-foundation.org,
iommu@lists.linux-foundation.org, linuxppc-dev@lists.ozlabs.org,
Christoph Hellwig <hch@lst.de>,
David Gibson <david@gibson.dropbear.id.au>
Subject: Re: [RFC PATCH] virtio_ring: Use DMA API if guest memory is encrypted
Date: Mon, 3 Jun 2019 21:42:58 -0400 [thread overview]
Message-ID: <20190603211528-mutt-send-email-mst@kernel.org> (raw)
In-Reply-To: <877ea26tk8.fsf@morokweng.localdomain>
On Mon, Jun 03, 2019 at 10:13:59PM -0300, Thiago Jung Bauermann wrote:
>
>
> Michael S. Tsirkin <mst@redhat.com> writes:
>
> > On Wed, Apr 17, 2019 at 06:42:00PM -0300, Thiago Jung Bauermann wrote:
> >> I rephrased it in terms of address translation. What do you think of
> >> this version? The flag name is slightly different too:
> >>
> >>
> >> VIRTIO_F_ACCESS_PLATFORM_NO_TRANSLATION This feature has the same
> >> meaning as VIRTIO_F_ACCESS_PLATFORM both when set and when not set,
> >> with the exception that address translation is guaranteed to be
> >> unnecessary when accessing memory addresses supplied to the device
> >> by the driver. Which is to say, the device will always use physical
> >> addresses matching addresses used by the driver (typically meaning
> >> physical addresses used by the CPU) and not translated further. This
> >> flag should be set by the guest if offered, but to allow for
> >> backward-compatibility device implementations allow for it to be
> >> left unset by the guest. It is an error to set both this flag and
> >> VIRTIO_F_ACCESS_PLATFORM.
> >
> >
> > OK so VIRTIO_F_ACCESS_PLATFORM is designed to allow unpriveledged
> > drivers. This is why devices fail when it's not negotiated.
>
> Just to clarify, what do you mean by unprivileged drivers? Is it drivers
> implemented in guest userspace such as with VFIO? Or unprivileged in
> some other sense such as needing to use bounce buffers for some reason?
I had drivers in guest userspace in mind.
> > This confuses me.
> > If driver is unpriveledged then what happens with this flag?
> > It can supply any address it wants. Will that corrupt kernel
> > memory?
>
> Not needing address translation doesn't necessarily mean that there's no
> IOMMU. On powerpc we don't use VIRTIO_F_ACCESS_PLATFORM but there's
> always an IOMMU present. And we also support VFIO drivers. The VFIO API
> for pseries (sPAPR section in Documentation/vfio.txt) has extra ioctls
> to program the IOMMU.
>
> For our use case, we don't need address translation because we set up an
> identity mapping in the IOMMU so that the device can use guest physical
> addresses.
And can it access any guest physical address?
> If the guest kernel is concerned that an unprivileged driver could
> jeopardize its integrity it should not negotiate this feature flag.
Unfortunately flag negotiation is done through config space
and so can be overwritten by the driver.
> Perhaps there should be a note about this in the flag definition? This
> concern is platform-dependant though. I don't believe it's an issue in
> pseries.
Again ACCESS_PLATFORM has a pretty open definition. It does actually
say it's all up to the platform.
Specifically how will VIRTIO_F_ACCESS_PLATFORM_NO_TRANSLATION be
implemented portably? virtio has no portable way to know
whether DMA API bypasses translation.
> --
> Thiago Jung Bauermann
> IBM Linux Technology Center
next prev parent reply other threads:[~2019-06-04 1:44 UTC|newest]
Thread overview: 39+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <87zhrj8kcp.fsf@morokweng.localdomain>
2019-01-29 17:42 ` [RFC PATCH] virtio_ring: Use DMA API if guest memory is encrypted Thiago Jung Bauermann
2019-01-29 19:02 ` Michael S. Tsirkin
2019-01-30 2:24 ` Jason Wang
2019-01-30 2:36 ` Michael S. Tsirkin
2019-01-30 3:05 ` Jason Wang
2019-01-30 3:26 ` Michael S. Tsirkin
2019-01-30 7:44 ` Christoph Hellwig
2019-02-04 18:15 ` Thiago Jung Bauermann
2019-02-04 21:38 ` Michael S. Tsirkin
2019-02-05 7:24 ` Christoph Hellwig
2019-02-05 16:13 ` Michael S. Tsirkin
2019-03-26 16:53 ` Michael S. Tsirkin
2019-02-04 18:14 ` Thiago Jung Bauermann
2019-02-04 20:23 ` Michael S. Tsirkin
2019-03-20 16:13 ` Thiago Jung Bauermann
2019-03-20 21:17 ` Michael S. Tsirkin
2019-03-22 0:05 ` Thiago Jung Bauermann
2019-03-23 21:01 ` Michael S. Tsirkin
2019-03-25 0:57 ` David Gibson
2019-04-17 21:42 ` Thiago Jung Bauermann
2019-04-17 21:42 ` Thiago Jung Bauermann
2019-04-19 23:09 ` Michael S. Tsirkin
2019-04-25 1:01 ` Thiago Jung Bauermann
2019-04-25 1:18 ` Michael S. Tsirkin
2019-04-26 23:56 ` Thiago Jung Bauermann
2019-05-20 13:08 ` Michael S. Tsirkin
2019-05-20 13:16 ` Michael S. Tsirkin
2019-06-04 1:13 ` Thiago Jung Bauermann
2019-06-04 1:42 ` Michael S. Tsirkin [this message]
2019-06-28 1:58 ` Thiago Jung Bauermann
2019-07-01 14:17 ` Michael S. Tsirkin
2019-07-14 5:51 ` Thiago Jung Bauermann
2019-07-15 14:35 ` Michael S. Tsirkin
2019-07-15 20:29 ` Thiago Jung Bauermann
2019-07-15 20:36 ` Michael S. Tsirkin
2019-07-15 22:03 ` Thiago Jung Bauermann
2019-07-15 22:16 ` Michael S. Tsirkin
2019-07-15 23:05 ` Thiago Jung Bauermann
2019-07-15 23:24 ` Benjamin Herrenschmidt
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